The presence of a mechanism which monitors the integrity of the genome and leads to cell cycle arrest in the event of DNA damage has been described as checkpoint control. Failure to arrest the cell cycle when DNA is damaged can lead to the propagation of mutations or loss of chromosomal material. While the phenomena of cell cycle arrest before DNA replication (G1 arrest) or after DNA replication (G2 arrest) is well- established, the signal transduction pathway which couples the detection of DNA damage to control of progression through the cell cycle is only beginning to be elucidated. The proposed project aims to characterize the checkpoint which leads to cell cycle arrest prior to mitosis in response to DNA damage, initially through the characterization of the chk1 protein kinase (p56chk1) in Schizosaccharomyces pombe. Cells which lack a functional chk1 gene fail to arrest the cell cycle prior to mitosis when DNA damage takes place, attempt to proceed through the cell cycle with damaged DNA and subsequently die. The products of several other genes are also required for cell cycle arrest in response to DNA damage; however, despite knowledge of the primary sequence of the rad gene products, little is known about their function. As a protein kinase, p56chk1 is an ideal protein with which to initiate a mechanistic characterization of this pathway. To determine the role of p56chk1 in mediating cell cycle arrest following DNA damage, two complementary lines of research will be carried out: biochemical studies to characterize the regulation of the activity of p56chk1 and genetic screens to identify proteins with which p56chk1 interacts. The fission and budding yeasts have been used successfully as tools for the identification of human genes which govern progression through the cell cycle due to evolutionary conservation of the molecules involved. The architecture of many signal transduction pathways has been conserved as well. Thus, characterization of the p56chk1 dependent cell cycle arrest pathway should prove to be a useful paradigm for dissecting the pathway coupling detection of DNA damage to cell cycle arrest in human cells. Information regarding the mechanism of cell cycle arrest will be vital for the design of more effective ways of overriding the pathway for chemotherapeutic treatment of tumor cells.